Screens of color television receivers have been rapidly upsized, as the satellite broadcasting service, video disc apparatus, S-VHS type VCRs, regular type VCRs, a rental video disc service and the like come into wide use in recent years.
In this context, screens of direct view CRT-type television receivers have been upsized to around a 30-inch diagonal screen or more. However, the weight of television receivers abruptly increases, as the diagonal screen size exceeds over the 30-inches. Thus, the direct view CRT-type television receivers with such 30-inch diagonal screens have reached a limit of practical use.
On the contrary, projection color television receivers can be designed at a relatively compact in size, as an image on a picture tube (CRT) is projected onto a screen by enlarging the image through lenses. Due to such a merit, the projection color television receivers dominate in television receivers with screens in the diagonal size exceeding over 40-inches.
In such projection color television receivers, red (R), green (G) and blue (B) monochrome projection picture tubes are aligned in a row with each other. Then picture images on these projection picture tubes are projected on a display screen by enlarging the images through an optical system comprised of lenses, reflecting mirrors and the like.
In the projection type color television receivers with the construction as described above, a convergence correction apparatus is normally provided for correcting a color discrepancy and a color irregularity caused by orbital offsets of electron beams resulting from a misalignment among the R, G and B monochrome projection picture tubes and an effect of the earth magnetic field to the electron beams.
In FIGS. 1, FIG. 1(a) shows a side view of a projection color television receiver, while FIG 1(b) shows a plan view illustrating an aligning state of the projection picture tubes. In FIG. 1(b), R, G and B monochrome projection picture tubes 21(a), 21(b) and 21(c) are aligned in a row with each other. Further, the R projection picture tube 21(a) and the B projection picture tube 21(c) are directed inwardly in connection with an arrangement of optical paths of their optical systems.
A projection picture tube 22 is associated with each of the R, G and B projection picture tubes 21(a), 21(b) and 21(c). Thereby picture images on the R, G and B projection picture tubes 21(a), 21(b) and 21(c) are projected on a display screen 24 by being enlarged through the projection lenses 22 and further a set of reflecting mirrors 23.
Generally, in order to adjust color discrepancies and/or color irregularities of picture images on a display screen, the picture image is adjusted its convergence characteristics by checking cross hatching patterns projected on the display screen from cross hatching pattern generators respectively associated with the R, G and B monochrome projection picture tubes 21(a), 21(b) and 21(c). The states of the cross hatching patterns on the R, G and B monochrome projection picture tubes 21(a), 21(b) and 21(c) without any convergence correction are shown in FIGS. 2(a), 2(b) and 2(c), respectively.
R, G and B monochrome beams radiated from the R, G, B projection picture tubes 21(a), 21(b) and 21(c) are applied on the screen 24 through the projection lenses 22 and the reflecting mirrors 23. Those projected images on the screen are distorted as illustrated in FIGS. 2(a), 2(b) and 2(c) due to mainly each attribute of the optical systems.
Such a convergence correction apparatus is used for electrically correcting optical distortions of lenses. For example the convergence correction apparatus is constructed as shown in FIGS. 3(a) and 3(b). FIGS. 3(a) and 3(b) typically show such a circuit for the R (red) projection picture tube 21(a), while circuits for the G (green) and the B (blue) projection picture tubes are also constructed in a similar manner the circuit as shown in FIGS. 3(a) and 3(b).
In FIG. 3, the reference numeral 1 denotes an antenna, the reference numeral 2 denotes a tuner, the reference numeral 3 denotes a detector/amplifier means, the reference numeral 4 denotes an integrated circuit (hereinafter referred as to IC) 4 for a video chroma processing, and the reference numeral 14 denotes a driving circuit for amplifying powers of R, G and B signals from the IC 4 and supplying them to the R, G and B projection picture tubes 21(a), 21(b) and 21(c), respectively.
A horizontal driving pulse H output from a synchronization/deflection processing circuit in the IC 4 is supplied to a horizontal driving circuit 32 in a synchronization/deflection circuit 30. The output of the horizontal driving circuit 32 is then supplied to a horizontal output circuit 33.
The output of the horizontal output circuit 33 is supplied to a horizontal deflection coil 100 and also to a horizontal output transformer 34. The output of the horizontal output transformer 34 is then supplied to an intermediate voltage circuit and a low voltage circuit (not shown) and the like.
Further, a horizontal periodic driving pulse signal HD is supplied to a convergence correction circuit 40 from the horizontal output transformer 34.
On the other hand, a vertical driving pulse signal V is supplied to a vertical output circuit 31. The output of this vertical output circuit 31 is then supplied to a vertical deflection coil 100. Further, a vertical periodic driving pulse signal VD is supplied from the vertical output circuit 31 to the convergence correction circuit 40.
The convergence correction circuit 40 is composed of a correction signal generator 41 which generates a horizontal sawtooth wave, a vertical sawtooth wave and a parabolic wave based on the horizontal pulse signal HD and the vertical pulse signal VD, and a matrix circuit 42 which generates signals for correcting various raster distortions by appropriately combining some or all of the horizontal periodic sawtooth wave, the vertical periodic sawtooth wave and the parabolic wave.
The output of this convergence correction circuit 40 is supplied to a driving amplifier 50 which drives the R, G, B projection picture tubes 21(a), 21(b) and 21(c). The output of the driving amplifier 50 is supplied to each convergence coil 101 provided for the R, G, B projection picture tubes 21(a), 21(b) and 21(c). In practical applications, such a driving amplifier is needed for each of horizontal and vertical deflections of the projection picture tubes. Thus six amplifiers are required in total for all of the R, G, B projection picture tubes 21(a), 21(b) and 21(c))
The horizontal driving pulse signal H is also supplied to a high voltage generator 60. The high voltage output from the high voltage generator 60 is then obtained via a flyback transformer 63 based on the horizontal driving pulse H. This high voltage output is supplied to each anode 26 of the R, G, B projection picture tubes 21(a), 21(b) and 21(c). This high voltage is stepped down by a voltage divider comprising resistors R1 and R2. The divided voltage is then supplied to a comparator 71. Corresponding to fluctuations of the high voltage, a high voltage fluctuating component obtained by comparing with a reference voltage in the comparator 71 is supplied to a high voltage controller 75 comprising a control transistor, a saturable reactor and the like, so that a voltage across the primary coil of the flyback transformer 63 is controlled in response to the high voltage fluctuation. The high voltage controller 75 and a high voltage fluctuation detector 80 comprised of the resistor type voltage divider and the compsrator 71 for comparing the divided voltage with the reference voltage constitute a voltage stabilizer.
By the way, in case of the conventional convergence correction apparatus, as shown in FIG. 3, there is such a problem that the high voltage stabilizer can not follow a rapid change of the brightness level on a display screen.
In particular, when an image data has a 100% brightness change from a black level to a white level, as illustrated in FIG. 4, a distortion of a picture image extending laterally to both sides due to a white band high voltage ripple is observed.
If it is attempted to correct image distortions on the screen only by means of convergence corrections, general optical distortions can be corrected in relatively easy. However, the distortion due to the white band high voltage ripple can not respond to the convergence correction means. As a result, three color beams of R, G and B can not be adequately superposed to each other on the display screen.
As described above, as to such a picture image distortion due to high voltage fluctuations, the high voltage fluctuation was attempted to remove by a high voltage stabilizer in a conventional convergence correction apparatus.
However, there was such a problem that the high voltage stabilizer could not fully respond to the high voltage fluctuation accompanied with a brightness change.
Further if it is attempted to correct a picture image distortion resulting from the high voltage fluctuation only by the convergence correction apparatus used for correcting general optical distortion corrections, the high voltage fluctuation resulting from the brightness change couldn't be corrected. Therefore, R, G and B color lights could not be superposed to each other on the display screen.